This invention relates generally to implantable defibrillators and more particularly to implantable defibrillator test circuits and methods.
Defibrillators of many kinds are known. In general, defibrillators apply electric pulses or shocks to the heart of a patient to stop an on-going fibrillation activity of a patient's heart. The fibrillation involves the generation of uncoordinated electrical signals in the heart which prevent the normal functioning and pumping of the heart. In some instances, the defibrillator is implanted subcutaneously in the body of the patient. The patient may also have a need for a pacemaker, to supply the heart with regular or intermittent pulses at predetermined locations of the heart, in order to complement or induce its regular working operation. There are products in the medical electronics field which combine the functions of defibrillation and pacing.
Once such a device is implanted, the patient nonetheless does not become immune to cardiac difficulties. For one reason or other, despite the implantation of the defibrillator into the body of the patient, circumstances may arise which require the use of an external defibrillator. The voltages applied during defibrillation, whether by an implanted device or an external device are very high. An example of a typical voltage level employed by an implanted defibrillator during defibrillation is 750 volts. For defibrillation by an external device, voltages of several thousand volts may be applied. When such a high voltage pulse is applied to the chest of a patient from an external device, and the patient has an implanted defibrillator, substantial damage may result to the implanted defibrillator. Accordingly, it is considered prudent to test the implanted defibrillator prior to its next use. Of particular concern is the high voltage output stage of the implanted defibrillator and the switches which isolate the defibrillation electrodes on the heart from the high voltage capacitors which are used to deliver the electric shock to the patient's heart. These switches have been known to fuse shut or to become inoperable in an open position. In either case, the switches are rendered useless in the regular operation of the implanted defibrillator.
Many patents have been granted in the field of defibrillation arrangements. For example, U.S. Pat. No. 5,111,816 has been issued to Ventritex and its inventors Benjamin Pless, John G. Ryan, and James M. Culp on a combined defibrillator/pacemaker. The patent shows a system which is able to detect defibrillator lead breaks without delivering a defibrillation pulse to the patient. U.S. Pat. No. 4,164,946 has been granted to Alois A. Langer on the subject of fault detection in a permanently implanted cardioverter or defibrillator. The circuitry of the Langer patent includes built-in interrogation and testing and complex fibrillation detection circuitry used to monitor an ECG signal and to issue a fibrillation detection signal when predetermined characteristics are detected. A characteristic output signal is produced when the fibrillation detector circuit is functioning properly.
Another recent patent, U.S. Pat. No. 5,224, 475 was granted to Berg et al. in 1993 on a method and apparatus for termination of ventricular tachycardia and ventricular fibrillation. According to this patent, an implantable defibrillator is provided with a plurality of defibrillation electrodes which may be reconfigured to define a plurality of defibrillation pathways. The device measures impedance along selected defibrillation pathways and monitors the success or failure of the pulse to accomplish defibrillation or cardioversion. The impedance paths are measured while electric current is applied to the heart of a patient. The teachings of this patent accordingly help optimize the correct positioning of electrodes in the body of a patient to ensure effective defibrillation can be accomplished. The impedance testing performed helps in the optimization process.
However, the patents indicated above are not specifically directed toward determination of the integrity of particular switching components in the defibrillation circuitry. These patents show approaches dealing with detection in lead breakage. They disclose complex arrangements to optimize the placement of defibrillation electrodes. The invention herein addresses something different.
In accordance with the invention, it is an object to ascertain the integrity of selected defibrillator components and switches to determine whether they are operating properly.
It is an object to test the circuitry of the defibrillator to determine whether any components are fused open or shut, thereby preventing their effective normal operation.
It is an object of the invention herein to conduct integrity testing without applying electric currents to or shocking the patient.
It is an object of the invention herein to inspect the condition of a defibrillator which is implanted in a patient without applying a defibrillation pulse to the patient.
It is further an object of the invention herein to detect the condition of implanted defibrillation circuitry without the use of fibrillation detection circuitry itself.
It is further an object of the invention herein to detect the condition of defibrillator circuitry with the use of the defibrillation circuitry normally used to implement defibrillation operation in a patient.